Differences in regional bone metabolism at the spine and hip: A quantitative study using ¹⁸F-fluoride positron emission tomography

Summary
This study showed that regional bone blood flow and ¹⁸F-fluoride bone plasma clearance measured by positron emission tomography are three times lower at the hip than the lumbar spine.

Introduction
Measurements of effective bone plasma flow (K ₁), bone plasma clearance (K _i ) and standardised uptake values (SUV) using ¹⁸F-fluoride positron emission tomography (¹⁸F-PET) provide a useful means of studying regional bone metabolism at different sites in the skeleton. This study compares the regional 18F-fluoride kinetics and SUV at the hip and lumbar spine (LS).

Methods
Twelve healthy postmenopausal women with no history of metabolic bone disease apart from two with untreated osteoporosis were recruited. Each subject underwent 60-min dynamic ¹⁸F-PET scans at the LS and proximal femur two weeks apart. K ₁, K _i and SUV were measured at the LS (mean of L₁–L₄), femoral neck (FN), total hip (TH) and femoral shaft (FS). Differences between sites were assessed using the nonparametric Kruskal–Wallis test with a Bonferroni correction for multiple comparisons.

Results
Values of K ₁, K _i and SUV at the FN, TH and FS were three times lower than at the LS (p = 0.003). Amongst the proximal femur sites, K _i and SUV were lower at the FS compared with the FN and TH, and SUV was lower at the TH compared with the FN (all p < 0.05). The volume of distribution was lower at the TH and FS compared with the LS (p < 0.05).

Conclusion
The lower values of K ₁, K _i and SUV at the hip suggest that lower bone blood flow in the proximal femur is an important factor explaining the principal reason for the differences in bone fluoride kinetics between the LS and hip sites.